Crab larval-release rhythms

Abstract

The same claim, worded differently, is made on the next page-clearly these authors meant what they said: they think the rhythms of these animals are not controlled by an internal clock. It is difficult to imagine how such a misunderstanding could arise: fiddler crabs and Carcinus are the classic organisms used in marine chronobiology studies. The first description of a clock-controlled tidal rhythm was done on a fiddler crab (Bennett e t al. 1957) and the animal has continued to be studied to the present day. Scores of papers on the rhythmicity of this crab, emanating from several labs, have been published (see the 34 yr summary of this work by Palmer 1991). The first study of the internal-clock control of Carcinus maenas rhythms was performed in 1958 (Naylor), and there are at least 50 more papers that have appeared since then from Naylor's lab and several others. The first study of the tidal rhythm of the fish Lipophrypho11s was done in 1965 (Gibson), and recently a nice series of papers (culminating in 1991) on the animal's rhythms have been published by Northcott, Gibson & Morgan; a review of the L. pholis rhythm, and the rhythmicity of other intertidal fishes, was published by Gibson (1992). There cannot be the slightest doubt in the minds of marine chronobiologists (other than Saigusa & Kawagoye) that the tide-associated rhythms of these 3 intertidal animals (and a great many others, see Palmer 1995) are under the control of internal living clocks. Further on in their paper, Saigusa & Kawagoye state that their results 'suggests [that] a light-response mechanism [is] involved even in the circatidal rhythm of intertidal animals' (p 92-93). This assertion is repeated again on p 94: 'This indicates that the 24-h LD cycle functions as the zeitgeber of the circatidal rhythm of Hemigrapsus sanguineus.' This claim creates a problem in my mind. Certainly (to give a n extreme example) one would not expect that the attendance of church each Sunday morning would set the phase of our second-to-second heart-beat rhythm. It is equally illogical to accept a conclusion that the 24-h day-night cycle would set the phase or entrain a n overt 12.4-h organismic tidal rhythm. If it did, the rhythm would no longer match the tides; it would become 24 h in length and lose the survival value once offered by a rhythm. Those 2 comments out of the way, I will turn directly to the rhythm itself. Larvae release in this species is a one-time event, i.e. each individual sheds all its larvae in one ca 6-h interval. This type of behavior is referred to in chronobiological literature a s a 'gated response', meaning it takes place only when a 'gate' is opened to allow the event. When a batch of crabs is collected and brought into the laboratory, they do not all release their offspring on the same day; instead, individuals release at different times over a n interval of several days. And when the response of an entire population, maintained in either a light-dark cycle or constant dim illumination, is plotted it is seen that individual releases occur roughly 25 h later each day. Taking the data at face value, the most obvious interpretation of the population-as-a-whole display is that it is a circa-